Eliminating the need for anodic gas separation in CO2 electroreduction systems via liquid-to-liquid anodic upgrading

Electrochemical reduction of CO2 to multi-carbon products (C2+), when powered using renewable electricity, offers a route to valuable chemicals and fuels. In conventional neutral-media CO2-to-C2+ devices, as much as 70% of input CO2 crosses the cell and mixes with oxygen produced at the anode. Recovering CO2 from this stream adds a significant energy penalty. Here we demonstrate that using a liquid-to-liquid anodic process enables the recovery of crossed-over CO2 via facile gas-liquid separation without additional energy input: the anode tail gas is directly fed into the cathodic input, along with fresh CO2 feedstock. We report a system exhibiting a low full-cell voltage of 1.9 V and total carbon efficiency of 48%, enabling 262 GJ/ton ethylene, a 46% reduction in energy intensity compared to state-of-art single-stage CO2-to-C2+ devices. The strategy is compatible with today’s highest-efficiency electrolyzers and CO2 catalysts that function optimally in neutral and alkaline electrolytes.

2 1 Fig. S1. The major mechanism of the electrochemical glucose oxidation reaction (GOR).  Table S1. The CO2 mass balance of the CO2RR-GOR system at 50 o C at equilibrium. F1, F2, F3 1 and F4 are the CO2 of input, crossover, converted and unreacted (Fig. 1d). All are normalized to 2 the equivalent volume flow rates of the CO2 gas. F2 was measured by GC with F1 of 10 sccm cm -3 2 (Fig. 3f in the main text), assuming it is unchanged with different input CO2 flow rates. F3 was 4 determined by analyzing the CO2RR products. F4 was measured by GC. Notably, F2 can be higher 5 than F1 at equilibrium because F2 is the 'dead' CO2 flow cycling from cathode to anode and back 6 to the cathode. NaOH scrubbing, causticization with lime, thermal calcination in a proposed oxy-blown kiln 400 ppm (air) 7.5 3 * These processes use liquefied natural gas to provide cryogenic energy. The energy cost for liquifying natural gas (LNG) to -162 o C (0.83 GJ/ton natural gas, 2.6 ton LNG per ton CO2) is added on top of the operating energy cost. Depending on the CO2 mole fraction, source of the input stream and the processes, capturing 1 ton 2 of CO2 in real-world plants requires 2.3~7.5 GJ. We select the best proximity of CO2 capture 3 energy based on the following considerations. 4 The typical capture processes in Table S2 include three types: monoethanolamine-based (1-3 in 5   Table S2), cryogenic-based (4-6 in Table S2) and inorganic alkali-based (7 and 8 in Table S2).

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Membrane and solid adsorbents-based approaches are less mature 11 . Monoethanolamine suffers 7 from the oxidative degradation induced by O2, and the CO2 mole fraction in the target gas input of 8 the alkali-based technique is far below that in the anodic gas stream (~66%). Therefore, we adopt 9 the CO2 separation energy of 3.4 GJ/ton CO2 (the lowest) in cryogenic-based approaches for 10 evaluating the anodic CO2 separation costs.

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Anodic gas stream CO2 capture economic assessment 14  Pt: 2 mg cm -2 . ECSAs of the Pt/C anode with various Pt loadings, measured following the protocol suggested in 6 the previous studies 14 . The single-component, three-electrode setup was used, with Pt/C on carbon 7 paper, graphite and Ag/AgCl as the working, counter and reference, respectively. The cyclic 2 mg cm -2 of Pt loading on the anode is close to the typical conventional IrO2 OER anode used in 1 CO2RR devices 15,16 . Considering that the market price of Ir is two-fold to Pt 17 , such a Pt loading 2 would not induce additional capital costs compared to the conventional CO2RR devices. In future, 3 non-precious-metal GOR catalysts 18 can be adopted in this system, reducing the associated 4 expenses.

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Increasing the anode Pt loading from 0.5 to 2.0 mg cm -2 lowers the cell voltage by up to 1.2 V ( Fig.   6 S5a), attributing to the increase of anodic active sites (reflected by the electrode geometric area 7 normalized ECSA, Fig. S5f) thus improved electrochemical kinetics. However, further increase 8 the loading to 3.2 mg cm -2 has insignificant effect to the cell voltage (Fig. S5a), despite its electrode 9 geometric area normalized ECSA is ~50% higher than 2.0 mg cm -2 (Fig. S5f). As such, for the 10 presented CO2RR-GOR system, we conclude that 2.0 mg cm -2 is sufficiently high. The cell voltage 11 is limited by other factors such as the glucose molecule mass transfer efficiency, ohmic resistance, The cell voltages of the electrolyzer using <1 mg cm -2 Cu : 2 (3.2) mg cm -2 Pt> are close to <0.5 6 mg cm -2 Cu : 2 mg cm -2 Pt> at the current density of 80 and 100 mA/cm 2 , but are higher at the 7 current densities over 120 mA cm -2 . The higher ohmic resistance of the former (1.92 vs. 1.20 ohm 8 cm 2 , Fig. S6) is responsible to its higher cell voltage.

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The electronic conductivity of the cathode relies on the continuous Cu-coated PTFE networks that 10 transfer electrons onto spray-coated Cu nanoparticles. The Cu nanoparticles we spray-coated on 11 the GDE forms a highly porous (non-continuous) layer, with PFSA ionomer coated on the particle 12 surface (see Fig. 2a in the main text). Therefore, the spray-coated Cu nanoparticle layer is less 13 conductive than the Cu-coated PTFE network, and a thicker Cu nanoparticle layer increases the 14 ohmic resistance. CO2RR can produce formate, migrate over the AEM and mix with that produced by GOR. The 5 specific catalyst in this study has a formate FE of 0.12% (Fig. S12a), negligible to the 5.6% 6 produced by GOR (Fig. S12b). We, therefore, assign the formate detected in the anolyte of the 7 CO2RR-GOR system to GOR. On the other hand, the other CO2RR productsethanol, propanol 8 and acetateare not detected in the GOR products.  glucarate and formate while the other potential products were below our limit of detection. We   The carbon originated from different sources has different 13 C abundances. For example, the 5 commercial CO2 has a δ 13 CVPDB value of -54‰ to -29‰, while the organisms have a δ 13 CVPDB 6 signature of about -25‰ 23,24 . We, therefore, measured the carbon isotopes in bicarbonate, CO2 in 7 the cylinder, glucose and the CO2 collected from the anodic gas stream to identify the source of 8 the CO2. As seen from the results listed in Table S4, the CO2 collected from the anodic gas stream 9 has a δ 13 CVPDB very close to that detected in bicarbonate. We thus conclude that the majority of 10 the anodic CO2 originates from the acidification of bicarbonate, of which the carbon was 11 supplemented by the crossover CO2. The contribution of the glucose over oxidation to the anodic 12 CO2if anyis below 0.2% (the measurement error for 13 C/ 12 C of the anodic CO2 gas sample is 13 ~7×10 -7 ). In light of the fact that: i) the sum of GOR FE from NMR and HPLC is close to 100%; 14 ii) the Pt/C catalyst we are using is inactive to the alcohols under the operating conditions; 15 iii) >99.8% of the anodic CO2 gas stream comes from bicarbonate acidification, we conclude that 16 the carbon efficiency and mass balance calculation in this work is valid and reliable.

Supplementary note 2:
1 The cathodic and anodic liquid-phase products are evaluated from the 1 H NMR spectra of catholyte 2 and anolyte, respectively. The typical 1 H NMR spectra are shown in Fig. S11. Notably, formate 3 can be detected in the anolyte which is majorly ascribed to the oxidation of glucose 25,26 . However, 4 some of the formate may also come from the CO2RR. In our previous studies 15 , the Cu 5 nanoparticle usually shows a low formate FE of < 1.5%.     (Fig. S13), accounting all these products, the total anodic FE is close to 100%.   (Fig. S13), accounting all these products, the total anodic FE is close to 100%.   (Fig. S13), accounting all these products, the total anodic FE is close to 100%.   (Fig. S13), accounting all these products, the total anodic FE is close to 100%.

Table S14
The comparison of the energy intensities between CO2RR-OER and CO2RR-GOR 1 systems operating at different current densities, based on the experimental results shown in Fig.   2 3d (CO2RR-GOR) and Fig. S3b (CO2RR-OER, the crossover CO2 was not recirculated). This 3 comparison assumes the CO2 in anodic gas stream in CO2RR-OER system is isolated from O2 then 4 redirected to the gas feeding since 33% O2 reduces the CO2RR selectivity to ~0%. The anodic gas 5 stream of the CO2RR-GOR system is co-fed with the input CO2 stream. The cathodic gas separation energy is not included because at a specific current density, the CO2RR-OER and CO2RR-GOR systems show similar ethylene FEs. We assume that their ethylene FE vs. carbon efficiency diagram have a similar trend, so as their cathodic separation requirements.  Electrochem. commun. 7, 189-193 (2005).